1. Field of the Invention
The present invention in general relates to internal combustion engines, a start of combustion in a cylinder of the internal combustion engine being ascertained from a cylinder-pressure curve that is provided.
2. Description of Related Art
During operation of internal combustion engines, combustion is able to be controlled in the cylinders. For this purpose, however, an analysis is required of the combustion processes taking place in the cylinders. An analysis of the combustion is preferably carried out by an evaluation of the curve of a cylinder pressure in the cylinder being observed. From this one is able to derive the start of combustion in the cylinder.
The curve of the cylinder pressure may be used, for instance, for calculating the heat release development, which describes the heat release caused by the combustion. Results of the analysis of a combustion in the cylinder are, for instance, the indicated average pressure, the heat developed, the start of combustion, the end of combustion and the duration of combustion.
Methods are known, for instance, from published German patent document DE 102 004 033 072, which define the start of combustion in such a way that the heating curve or the cumulative heating curve exceeds a specified threshold, e.g. 5% of the overall heat liberated. However, a determination of the start of combustion cannot always be made without doubt from the heating curve, especially in the case of small injection quantities.
Among the abovementioned results of combustion analysis, the determination of the start of combustion represents an important measure for improving combustion control in the cylinders. In particular, the start of combustion detection is used for the following purposes:    1. The monitoring of the start of injection will be required in the future by legal regulations. Malfunctions of the injection system may be detected thereby, in which the system is not in a position of supplying fuel at a certain crankshaft angle that is necessary to keep pollutant emission at or below a specified level. From the start of combustion, the start of injection φEB=φBB−φZV (φEB: crankshaft angle of a start of injection, φBB: crankshaft angle of a start of combustion, and φZV: crankshaft angle of an ignition delay) is able to be ascertained and monitored. This enables one to detect an injection that is too early (φEB<φEB—setpoint) or too late (φEB>φEB—setpoint). This assumes that the ignition delay φZV, which is a function of a parameter vector θ, is known sufficiently well or is calculated using an ignition delay model.            The deviations of the actual start of combustion from the setpoint start of combustion may be supplied to a controller which shifts the start of control for the injection valve or valves appropriately and/or displays a fault message to the driver and/or stores an appropriate information in a fault memory for diagnosis in the repair shop.            2. Furthermore, if the start of combustion is known, the ignition delay φZV=φBB−φEB is able to be monitored. With that, changes in the cylinder charge (e.g. oxygen content or residual gas content, temperature) or the ignitability of the fuel (cetane number), which lead to a more rapid (φZV<φZV—setpoint) or a slower (φZV>φZV—setpoint) inflammation, are able to be detected and compensated for. This presumes that the start of injection τEB is known sufficiently well. Start of injection φEB may be calculated from the start of control and the injection delay (the elapsed time between the electrical control and the opening of the nozzle) (φEB=φAB+φEV≈φAB+n*τEV) (φAB: crankshaft angle at start of control). Injection delay φEV is generally proportional to rotational speed n on the assumption of a constant injection delay in time (τEV≈constant). Deviations of the actual ignition delay from setpoint ignition delay φZV—setpoint may be supplied to a controller which correspondingly adjusts operating parameters such as start of control, rail pressure or charge pressure and/or displays it to the driver (as a fault message) and/or stores it in a fault memory for diagnosis in the repair shop.    3. For regeneration in response to active exhaust gas aftertreatment, post-injections are used, among other things, whose purpose is to increase the exhaust gas temperature. Since those post-injections are able to reinforce an unfavorable thinning of oil on the cylinder wall, it is necessary to select the start of injection and the start of combustion as late as possible with respect to the temperature increase and as early as possible with respect to the extent of the thinning of the oil. This requires the regulation of the start of combustion to a defined point in time, which takes into account the divergent aims named above. For this purpose, a setpoint value that is a function of an operating point is generally determined for the start of combustion, to which the actual start of combustion is regulated.
Up to now, a series of possibilities has been known for determining the start of combustion from the cylinder pressure curve that is provided. The start of combustion generally corresponds to the first clear pressure increase in the cylinder, or is defined as the end of the pure compression phase in the cylinder.
One possibility of ascertaining the start of combustion from the cylinder pressure curve is to subdivide the cylinder pressure curve into a compression portion and a combustion portion and to derive from this the start of combustion. This is known, for example, from published German patent document DE 10 2005 026 724. It is described there that the cylinder pressure curve is recalculated to a logarithmic transformation curve shape.
The document of Assanis, D. N.; Filipi, Z. S.; Fiveland, S. B; Syrimis, M.: “A predictive ignition delay correlation under steady-state and transient operation of a direct-injection Diesel engine”, ASME-ICE Fall Technical Conference, Ann Arbor, Mich., 1999, proposes the utilization of the maximum of the second derivative of the cylinder pressure curve as a criterion for the start of combustion. A similar procedure is described in the document of Katrasnik, T. et al., “A new criterion to determine the start of combustion in Diesel engines”, Journal of Engineering for Gas Turbines and Power, No. 4, pp. 928-933, the authors favoring the maximum of the third derivative as the point in time of the start of combustion.
It is proposed in U.S. Pat. No. 6,840,218 to submit the cylinder pressure curve to a wavelet transformation which makes possible a temporal assignment of occurring frequencies.
A sudden increase in the absolute value of the wavelet coefficients is drawn upon, in this instance, as an indicator for the point in time of the start of combustion.
Methods which undertake a determination of the times of one or more starts of combustion, from the combustion curve or the heating curve, do not possess sufficient robustness for the secure detection of the ignition delay in the entire engine operating plane. Problems occur, above all, at working points having low injection quantities, as well as at signal interferences of the cylinder pressure signal. These problems are avoided in the above methods, which utilize the cylinder pressure signal directly.
It is an object of the present invention to provide a method and a device for ascertaining one or more starts of combustion from a provided cylinder pressure curve.